Method for transmitting ppdu in wireless local area network and apparatus for the same

ABSTRACT

A method and device for receiving a data block in a wireless communication system, the method performed by a receiver. The method includes: receiving a physical layer protocol data unit (PPDU) from a transmitter over an operating channel, the PPDU including a signal field, a Very High Throughput-Signal-A (VHT-SIG-A) field, a Very High Throughput-Signal-B (VHT-SIG-B) field and a padded data block, generating a first data block by removing zero or more physical padding bits from the padded data block in a physical layer; and generating a second data block by removing zero or more Medium Access Control (MAC) padding bits from the first data block in a MAC layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. §119(e), this application claims the benefit ofpriority from U.S. Provisional Patent Application Ser. No. 61/322,271filed on Apr. 8, 2010, the contents of which is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless local area network (WLAN)system, and more particularly, to a method and apparatus fortransmitting a physical layer convergence procedure (PLCP) protocol dataunit (PPDU).

2. Related Art

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

Ever since the institute of electrical and electronics engineers (IEEE)802, i.e., a standardization organization for WLAN technologies, wasestablished in February 1980, many standardization works have beenconducted.

In the initial WLAN technology, a frequency of 2.4 GHz was usedaccording to the IEEE 802.11 to support a data rate of 1 to 2 Mbps byusing frequency hopping, spread spectrum, infrared communication, etc.Recently, the WLAN technology can support a data rate of up to 54 Mbpsby using orthogonal frequency division multiplex (OFDM). In addition,the IEEE 802.11 is developing or commercializing standards of varioustechnologies such as quality of service (QoS) improvement, access pointprotocol compatibility, security enhancement, radio resourcemeasurement, wireless access in vehicular environments, fast roaming,mesh networks, inter-working with external networks, wireless networkmanagement, etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to above 540 Mbps, and is based on amultiple input and multiple output (MIMO) technique which uses multipleantennas in both a transmitter and a receiver to minimize a transmissionerror and to optimize a data rate. In addition, this standard may use acoding scheme which transmits several duplicate copies to increase datareliability and also may use the OFDM to support a higher data rate.

An IEEE 802.11n HT WLAN system employs an HT green field physical layerconvergence procedure (PLCP) protocol data unit (PPDU) format which is aPPDU format designed effectively for an HT station (STA) and which canbe used in a system consisting of only HT STAs supporting IEEE 802.11nin addition to a PPDU format supporting a legacy STA. In addition, anHT-mixed PPDU format which is a PPDU format defined such that a systemin which the legacy STA and the HT STA coexist can support an HT system.

With the widespread use of a wireless local area network (WLAN) and thediversification of applications using the WLAN, there is a recent demandfor a new WLAN system to support a higher throughput than a dataprocessing rate supported by the IEEE 802.11n. A next generation WLANsystem supporting a very high throughput (VHT) is a next version of theIEEE 802.11n WLAN system, and is one of IEEE 802.11 WLAN systems whichhave recently been proposed to support a data processing rate of 1 Gbpsor higher in a medium access control (MAC) service access point (SAP).

The next generation WLAN system allows simultaneous channel access of aplurality of VHT-STAs for the effective use of a radio channel. Forthis, a multi-user multiple input multiple output (MU-MIMO)-basedtransmission using multiple antennas is supported. A VHT-access point(AP) can perform spatial division multiple access (SDMA) transmissionfor transmitting spatial-multiplexed data to the plurality of VHT-STAs.When data is simultaneously transmitted by distributing a plurality ofspatial streams to the plurality of STAs by the use of a plurality ofantennas, an overall throughput of the WLAN system can be increased.

When using the MU-MIMO transmission method for transmitting datasimultaneously with respect to the plurality of VHT-STAs, a physicallayer convergence procedure (PLCP) protocol data unit (PPDU) includingdata intended to be transmitted to the plurality of VHT-STAs paired withthe VHT-AP is transmitted through orthogonal frequency divisionmultiplexing (OFDM) symbols, and the number of OFDM symbols is identicalfor each VTH-STA. However, a size of data intended to be transmitted toeach VHT-STA may differ. Therefore, there is a need for a method forgenerating and transmitting a PPDU in such a manner that a specificpadding bit is appended according to a size of data transmitted to aVHT-STA so that PPDUs to be transmitted to a plurality of STAs have thesame bit size and thus are transmitted through the same number of OFDMsymbols.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for transmitting aphysical layer convergence procedure (PLCP) protocol data unit (PPDU) ina wireless local area system (WLAN) system.

In an aspect, a method of transmitting a data block in a wirelesscommunication system is provided. The method includes generating a datablock used for multi-user transmission, the data block comprising afirst control field and a data field for a plurality of users, the firstcontrol field indicating multi-user transmission of the data block, thedata field comprising a plurality of data units, each data unitcomprising a PSDU(PLCP(Physical Layer Convergence Procedure) ServiceData Unit) for each user and padding bits which are appended to the PSDUfor each user; and transmitting the data block to the plurality ofusers; wherein the number of the padding bits in each data unit isdetermined so that lengths of the plurality of data units are same andthe number of the padding bits in each data unit is determined byN_(sym), N_(DBPS,k), N_(PSDU,k), where N_(sym) denotes the number ofOFDM symbols of the data field, N_(DBPS,k) denotes the number of databits per OFDM symbol for k-th user, and the N_(PSDU,k) denotes thelength of the PSDU for k-th user.

Each data unit may further includes a service field indicating ascramble sequence used for scambling the PSDU for each user and thepadding bits, and the number of the padding bits in each data unit isdetermined by N_(sym), N_(DBPS,k), N_(PSDU,k), where N_(service) denotesa length of the service field.

Each data unit may further includes a tail field indicating tail bitsused for encoding the data field.

The value of N_(sym) may be same for the plurality of users.

The method may further include determining a number of OFDM symbolsN_(sym,k) used for transmission of a PSDU for k-th user; and selecting amaximum value among the plurality of N_(sym,k) as the N_(sym).

The number of the padding bits may be between 0 and 7 inclusive.

The data block may further include plurality of second control field forthe plurality of users, each second control field indicating a length ofthe PSDU for each user.

The data block may be a PPDU(PLCP Protocol Data Unit).

In another aspect, a wireless apparatus is provided. The apparatusincludes a processor; and a transceiver operationally coupled to theprocessor to transmit and receive a data block, wherein the processor isconfigured for: generating a data block used for multi-usertransmission, the data block comprising a first control field and a datafield for a plurality of users, the first control field indicatingmulti-user transmission of the data block, the data field comprising aplurality of data units, each data unit comprising a PSDU(PLCP(PhysicalLayer Convergence Procedure) Service Data Unit) for each user andpadding bits which are appended to the PSDU for each user; andtransmitting the data block to the plurality of users; wherein thenumber of the padding bits in each data unit is determined so thatlengths of the plurality of data units are same and the number of thepadding bits in each data unit is determined by N_(sym), N_(DBPS,k),N_(PSDU,k), where N_(sym) denotes the number of OFDM symbols of the datafield, N_(DBPS,k) denotes the number of data bits per OFDM symbol fork-th user, and the N_(PSDU,k) denotes the length of the PSDU for k-thuser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a physical layer (PHY) architecture of the institute ofelectrical and electronics engineers (IEEE) 802.11.

FIG. 2 is a diagram showing an example of a physical layer convergenceprocedure (PLCP) protocol data unit (PPDU) format used in a wirelesslocal area network (WLAN) system based on the IEEE 802.11n standard.

FIG. 3 shows an orthogonal mapping matrix based on a channel layer.

FIG. 4 shows an example of a high throughput (HT)-mixed PPDU format.

FIG. 5 shows an example of a PPDU format used in a next generation WLANsystem.

FIG. 6 shows a PPDU format according to an embodiment of the presentinvention.

FIG. 7 shows an example of a transmission (Tx) procedure based on a PPDUgeneration method according to an embodiment of the present invention.

FIG. 8 shows an example of a reception (Rx) procedure for a generatedPPDU according to an embodiment of the present invention.

FIG. 9 is a block diagram showing a wireless apparatus for implementingan embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings.

A wireless local area network (WLAN) system according to an embodimentof the present invention includes at least one basic service set (BSS).The BSS is a set of stations (STAs) successfully synchronized tocommunicate with one another. The BSS can be classified into anindependent BSS (IBSS) and an infrastructure BSS.

The BSS includes at least one STA and an access point (AP). The AP is afunctional medium for providing a connection to STAs in the BSS throughrespective wireless media. The AP can also be referred to as otherterminologies such as a centralized controller, a base station (BS), ascheduler, etc.

The STA is any functional medium including a medium access control (MAC)and wireless-medium physical layer (PHY) interface satisfying theinstitute of electrical and electronics engineers (IEEE) 802.11standard. The STA may be an AP or a non-AP STA. Hereinafter, the STArefers to the non-AP STA unless specified otherwise.

The STA can be classified into a very high throughput (VHT)-STA, a highthroughput (HT)-STA, and a legacy (L)-STA. The HT-STA is an STAsupporting IEEE 802.11n. The L-STA is an STA supporting a previousversion of IEEE 802.11n, for example, IEEE 802.11a/b/g. The L-STA isalso referred to as a non-HT STA.

FIG. 1 shows an IEEE 802.11 physical layer (PHY) architecture.

The IEEE 802.11 PHY architecture includes a PHY layer management entity(PLME), a physical layer convergence procedure (PLCP) sub-layer 110, anda physical medium dependent (PMD) sub-layer 100. The PLME provides a PHYmanagement function in cooperation with a MAC layer management entity(MLME). The PLCP sub-layer 110 located between a MAC sub-layer 120 andthe PMD sub-layer 100 delivers to the PMD sub-layer 100 a MAC protocoldata unit (MPDU) received from the MAC sub-layer 120 under theinstruction of the MAC layer, or delivers to the MAC sub-layer 120 aframe received from the PMD sub-layer 100. The PMD sub-layer 100 is alower layer of the PDCP sub-layer and serves to enable transmission andreception of a PHY entity between two STAs through a radio medium. TheMPDU delivered by the MAC sub-layer 120 is referred to as a physicalservice data unit (PSDU) in the PLCP sub-layer 110. Although the MPDU issimilar to the PSDU, when an aggregated MPDU (A-MPDU) in which aplurality of MPDUs are aggregated is delivered, individual MPDUs andPSDUs may be different from each other.

The PLCP sub-layer 110 attaches an additional field includinginformation required by a PHY transceiver to the MPDU in a process ofreceiving the MPDU from the MAC sub-layer 120 and delivering a PSDU tothe PMD sub-layer 100. The additional field attached in this case may bea PLCP preamble, a PLCP header, tail bits required on a data field, etc.The PLCP preamble serves to allow a receiver to prepare asynchronization function and antenna diversity before the PSDU istransmitted. The PLCP header includes a field that contains informationon a PLCP protocol data unit (PDU) to be transmitted, which will bedescribed below in greater detail with reference to FIG. 2.

The PLCP sub-layer 110 generates a PLCP protocol data unit (PPDU) byattaching the aforementioned field to the PSDU and transmits thegenerated PPDU to a reception STA via the PMD sub-layer. The receptionSTA receives the PPDU, acquires information required for data recoveryfrom the PLCP preamble and the PLCP header, and recovers the data.

FIG. 2 is a diagram showing an example of a PPDU format used in a WLANsystem based on the IEEE 802.11n standard.

Referring to FIG. 2, there are three types of PPDUs supported in IEEE802.11n.

FIG. 2( a) shows a legacy PPDU (L-PPDU) format for a PPDU used in theexisting IEEE 802.11a/b/g. Therefore, an L-STA can transmit and receivean L-PPDU having this format in a WLAN system based on the IEEE 802.11nstandard.

An L-PPDU 210 includes an L-STF field 211, an L-LTF field 212, an L-SIGfield 213, and a data field 214.

The L-STF field 211 is used for frame timing acquisition, automatic gaincontrol (AGC) convergence, coarse frequency acquisition, etc.

The L-LTF field 212 is used for frequency offset and channel estimation.

The L-SIG field 213 includes control information for demodulation anddecoding of the data field 214.

The L-PPDU may be transmitted in the order of the L-STF field 211, theL-LTF field 212, the L-SIG field 213, and the data field 214.

FIG. 2( b) is a diagram showing an HT-mixed PPDU format in which anL-STA and an HT-STA can coexist. An HT-mixed PPDU 220 includes an L-STFfield 221, an L-LTF field 222, an L-SIG field 223, an HT-SIG field 224,an HT-STF field 225, a plurality of HT-LTF fields 226, and a data field227.

The L-STF field 221, the L-LTF field 222, and the L-SIG field 223 areidentical to those shown in FIG. 2( a). Therefore, the L-STA caninterpret the data field by using the L-STF field 221, the L-LTF field222, and the L-SIG field 223 even if the HT-mixed PPDU 220 is received.The L-LTF field 222 may further include information for channelestimation to be performed by the HT-STA in order to receive theHT-mixed PPDU 220 and to interpret the L-SIG field 223, the HT-SIG field224, and the HT-STF field 225.

The HT-STA can know that the HT-mixed PPDU 220 is a PPDU dedicated tothe HT-STA by using the HT-SIG field 224 located next to the L-SIG field223, and thus can demodulate and decode the data field 227.

The HT-STF field 225 may be used for frame timing synchronization, AGCconvergence, etc., for the HT-STA.

The HT-LTF field 226 may be used for channel estimation for demodulationof the data field 227. Since the IEEE 802.11n supports single user-MIMO(SU-MIMO), a plurality of the HT-LTF fields 226 may be configured forchannel estimation for each of data fields transmitted through aplurality of spatial streams.

The HT-LTF field 226 may consist of a data HT-LTF used for channelestimation for a spatial stream and an extension HT-LTF additionallyused for full channel sounding. Therefore, the number of the pluralityof HT-LTF fields 226 may be equal to or greater than the number ofspatial streams to be transmitted.

The L-STF field 221, the L-LTF field 222, and the L-SIG field 223 aretransmitted first so that the L-STA also can acquire data by receivingthe HT-mixed PPDU 220. Thereafter, the HT-SIG field 224 is transmittedfor demodulation and decoding of data transmitted for the HT-STA.

Up to fields located before the HT-SIG field 224, transmission isperformed without beamforming so that the L-STA and the HT-STA canacquire data by receiving a corresponding PPDU. In the subsequentlyfields, i.e., the HT-STF field 225, the HT-LTF 226, and the data field227, radio signal transmission is performed by using precoding. In thiscase, the HT-STF field 225 is transmitted so that an STA that receives aprecoded signal can consider a varying part caused by the precoding, andthereafter the plurality of HT-LTF fields 226 and the data field 227 aretransmitted.

Even if an HT-STA that uses 20 MHz in an HT WLAN system uses 52 datasubcarriers per OFDM symbol, an L-STA that also uses 20 MHz uses 48 datasubcarriers per OFDM symbol. Since the HT-SIG field 224 is decoded byusing the L-LTF field 222 in a format of the HT-mixed PPDU 220 tosupport backward compatibility, the HT-SIG field 224 consists of 48×2data subcarriers. The HT-STF field 225 and the HT-LTF 226 consist of 52data subcarriers per OFDM symbol. As a result, the HT-SIG field 224 issupported using ½ binary phase shift keying (BPSK), each HT-SIG field224 consists of 24 bits, and thus 48 bits are transmitted in total. Thatis, channel estimation for the L-SIG field 223 and the HT-SIG field 224is performed using the L-LTF field 222, and a bit sequence constitutingthe L-LTF field 222 can be expressed by Equation 1 below. The L-LTFfield 222 consists of 48 data subcarriers per one symbol, except for aDC subcarrier.

L _(−26,26)={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1}  [Equation 1]

FIG. 2( c) is a diagram showing a format of an HT-Greenfield PPDU 230that can be used by only an HT-STA. The HT-GF PPDU 230 includes anHT-GF-STF field 231, an HT-LTF1 field 232, an HT-SIG field 233, aplurality of HT-LTF2 fields 234, and a data field 235.

The HT-GF-STF field 231 is used for frame timing acquisition and AGC.

The HT-LTF1 field 232 is used for channel estimation.

The HT-SIG field 233 is used for demodulation and decoding of the datafield 235.

The HT-LTF2 234 is used for channel estimation for demodulation of thedata field 235. Since the HT-STA uses SU-MIMO, channel estimation isrequired for each of data fields transmitted through a plurality ofspatial streams, and thus a plurality of HT-LTF2 fields 234 may beconfigured.

The plurality of HT-LTF2 fields 234 may consist of a plurality of dataHT-LTFs and a plurality of extension HT-LTFs, similarly to the HT-LTF226 of the HT-mixed PPDU 220.

Each of the data fields 214, 227, and 235 respectively shown in FIGS. 2(a), (b), and (c) may include a service field, a scrambled PSDU field, atail bits field, and a padding bits field.

In order to use MIMO in a WLAN system supporting an HT, an HT-LTF isdefined for channel estimation. The HT-LTF is used for channelestimation similarly to an L-LTF, but has a difference in that theHT-LTF can estimate a MIMO channel. In order to estimate the MIMOchannel by using the HT-LTF, an orthogonal mapping matrix P_(HTLTF) isused by being multiplied by the HT-LTF. The P_(HTLTF) consists of ‘1’and ‘−1’ and can be expressed by Equation 2 below.

$\begin{matrix}{P_{HTLTF} = \begin{bmatrix}1 & {- 1} & 1 & 1 \\1 & 1 & {- 1} & 1 \\1 & 1 & 1 & {- 1} \\{- 1} & 1 & 1 & 1\end{bmatrix}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$

Herein, the orthogonal mapping matrix is used in a different sizeaccording to a channel layer, which will be described with reference toFIG. 3.

FIG. 3 shows an orthogonal mapping matrix based on a channel layer.

Referring to FIG. 3, a training symbol is defined on a spatial streambasis, and is transmitted for channel estimation of each spatial stream.When the number of spatial streams is 1, 2, and 4, the number of HT-LTFsto be transmitted is 1, 2, and 4, respectively. When the number ofspatial streams is 3, one extra long training symbol is used so that 4HT-LTFs can be used.

For SU-MIMO supported in the HT WLAN system, channel information betweena transmitter and a receiver needs to be known to an AP and/or an STAwhich intend to transmit and receive a PPDU by using a MIMO transmissionmethod. In order to report the channel information, the transmitter mayperform channel estimation. The channel estimation can be performedbased on training symbols of a physical layer convergence procedure(PLCP) preamble of a sounding PPDU. More specifically, the channelestimation can be performed based on the HT-LTF 226 shown in FIG. 2( b).The HT-LTF used in the channel estimation may be determined by the totalnumber of antennas of the receiver or the number of spatial streams tobe transmitted.

An HT-mixed PPDU format is a PPDU format for supporting a highthroughput (HT) in a WLAN system in which a legacy STA and an HT-STAcoexist. L-STF, L-LTF, and L-SIG fields are transmitted first so that anHT-mixed PPDU is also received by the legacy STA to obtain data throughdecoding and demodulation. Thereafter, an HT-SIG field is transmittedfor transmission of information required to demodulation and decoding ofdata transmitted for an HT. Until the HT-SIG field, transmission isperformed without beamforming so that various STAs including the legacySTA can receive this information. Regarding HT-LTF and data fields to betransmitted after the HT-SIG field, the HT-STF field is transmitted andthen the HT-LTF and data fields are transmitted so as to be able toconsider a part in which power varies by precoding.

As described above with reference to FIG. 2, the data field may includea service field, a PSDU, a tail bit field, and a padding bit field.Since the PPDU is transmitted by matching a specific number of bitsequences to a plurality of OFDM symbols, a bit-stream size of the PPDUmay be determined according to a specific rule. For this, the PPDU isgenerated by being appended with a padding bit sequence according to aspecific padding rule. An example of applying the padding rule will beexplained with reference to FIG. 4 showing the HT-mixed PPDU format.

FIG. 4 shows an example of an HT-mixed PPDU format.

Referring to FIG. 4, an HT-mixed PPDU 400 includes an L-STF field 410,an L-LTF field 420, an L-SIG field 430, an HT-SIG field 440, an HT-LTFfield 450, a service field 460, a PSDU field 470, a tail field 480, anda padding field 490. Since the L-STF field 410, the L-LTF field 420, theL-SIG field 430, the HT-SIG field 440, and the HT-LTF field 450 are thesame as those in FIG. 2( b), detailed explanations thereof will beomitted.

The service field 460 is used to initialize a scrambler. 16 bits may beallocated for the service field.

The tail field 480 may be configured with a bit sequence required toreturn a convolution encoder to a state ‘0’. A bit size allocated to thetail field may be in proportion to the number of binary convolutionalcode (BCC) encoders used to encode data to be transmitted. Moreparticularly, the bit size allocated to the tail field may be 6 bits×thenumber of BCC encoders.

The PSDU 470 may be a MAC protocol data unit (MPDU) or aggregate MPDU(A-MPDU) which is a data unit delivered from a MAC layer. A size of abit sequence constituting the PSDU may be expressed by a value of alength sub-field included in the HT-SIG field 440. The size of the bitsequence of the PSDU may be expressed on an octet basis.

The padding field 490 consists of bits for filling a bit space whichremains when a bit size to be allocated for each OFDM is not satisfiedeven if bits constituting the PSDU and a bit constituting the tail fieldare included in a last symbol among a plurality of OFDM symbolstransmitted by allocating the PPDU 400. A method of configuring thepadding field will be described in greater detail with reference toEquation 3 below.

$\begin{matrix}{N_{SYM} = \lceil \frac{\begin{matrix}( {{8 \times {{length}( {{of}\mspace{14mu} {the}\mspace{14mu} {PSDU}\mspace{14mu} {in}\mspace{14mu} {SIG}\mspace{14mu} {field}} )}} +}  \\{16 + {6 \times N_{BS}}}\end{matrix}}{N_{DBPS}} \rceil} & \lbrack {{Equation}\mspace{14mu} 3} \rbrack\end{matrix}$

Herein, N_(SYM) denotes a data field 40, i.e., the number of OFDMsymbols used to transmit a bit stream constituting the service field460, the PSDU 470, the tail field 480, and the padding field 490 to areceiving-end STA, N_(ES) denotes the number of BCC encoders for thedata field 40, and N_(DBPS) denotes the number of data bits allocatedfor each OFDM symbol. And, ┌x┐ denotes the smallest integer greater thanor equal to x.

The number ‘16’ in the numerator of Equation 3 denotes the number ofbits allocated to the service field 460. 6×N_(ES) denotes the number ofbits allocated to the tail field 480. N_(DBPS) may refer to Table 1 andTable 2 below. In Table 1, the number of spatial streams is 1. In Table2, the number of spatial streams is 2.

TABLE 1 Data rate (Mb/s) MCS 800 ns 400 ns GI Index Modulation RN_(BPSCS)(i_(SS)) N_(SD) N_(SP) N_(CBPS) N_(DBPS) GI (see NOTE) 0 BPSK ½1 52 4 52 26 6.5 7.2 1 QPSK ½ 2 52 4 104 52 13.0 14.4 2 QPSK ¾ 2 52 4104 78 19.5 21.7 3 16-QAM ½ 4 52 4 208 104 26.0 28.9 4 16-QAM ¾ 4 52 4208 156 39.0 43.3 5 64-QAM ⅔ 6 52 4 312 208 52.0 57.8 6 64-QAM ¾ 6 52 4312 234 58.5 65.0 7 64-QAM ⅚ 6 52 4 312 260 65.0 72.2 NOTE Support of400 ns GI is optional on transmit and receive.

TABLE 2 Data rate (Mb/s) MCS 800 ns 400 ns GI Index Modulation RN_(BPSCS)(i_(SS)) N_(SD) N_(SP) N_(CBPS) N_(DBPS) GI (see NOTE) 8 BPSK ½1 52 4 104 52 13.0 14.4 9 QPSK ½ 2 52 4 208 104 26.0 28.9 10 QPSK ¾ 2 524 208 156 39.0 43.3 11 16-QAM ½ 4 52 4 416 208 52.0 57.8 12 16-QAM ¾ 452 4 416 312 78.0 86.7 13 64-QAM ⅔ 6 52 4 624 416 104.0 115.6 14 64-QAM¾ 6 52 4 624 468 117.0 130.0 15 64-QAM ⅚ 6 52 4 624 520 130.0 144.4 NOTEThe 400 ns GI rate values are rounded to 1 decimal place.

Each symbol shown in Table 1 and Table 2 above may refer to Table 3below.

TABLE 3 Symbol Explanation N_(SS) Number of spatial streams R Codingrate N_(BPSC) Number of coded bits per single carrier (total acrossspatial streams) N_(BPSCS)(i_(SS)) Number of coded bits per singlecarrier for each spatial stream, i_(SS) = 1, . . . , N_(SS) N_(SD)Number of complex data numbers per spatial stream per OFDM symbol N_(SP)Number of pilot values per OFDM symbol N_(CBPS) Number of coded bits perOFDM symbol N_(DBPS) Number of data bits per OFDM symbol N_(ES) Numberof BCC encoders for the DATA field N_(TBPS) Total bits per subcarrier

As can be seen in Table 1 to Table 2, N_(DBPS) may have a value thatsatisfies Equation 4 below.

N _(DBPS) =N _(SD)×Mod×code rate×N _(SS)   [Equation 4]

According to Equations 3 and 4, the number of padding bits constitutingthe padding field 490 filled to a last OFDM symbol can be calculated,which can be expressed by Equation 5 below.

N _(PAD) =N _(SYM) ×N _(DBPS)−(8×length+16+6×N _(BS))   [Equation 5]

By using the aforementioned method of determining the padding bit(s) tobe appended, a PPDU format usable in the HT WLAN system can becompleted. In the above equation, the term ‘length’ denotes a length ofthe PSDU.

Unlike the IEEE 802.11n standard supporting the HT, a next generationWLAN system requires a higher throughput. This is called a very highthroughput (VHT) to distinguish it from the HT, and 80 MHz bandwidthtransmission and/or higher bandwidth transmission (e.g., 160 MHz) aresupported in the next generation WLAN system. In addition, multiuser-multiple input multiple output (MU-MIMO) transmission is supported.

An amount of control information transmitted to STAs for MU-MIMOtransmission may be relatively greater than an amount of IEEE 802.11ncontrol information. Examples of control information additionallyrequired for the next generation WLAN system may be informationindicating the number of spatial streams to be received by each STA,information regarding modulation and coding of data transmitted to eachSTA, etc. Therefore, when MU-MIMO transmission is performed in order toprovide data services simultaneously to a plurality of STAs, the amountof control information to be transmitted may be increased according tothe number of receiving STAs.

In order to effectively transmit the increased amount of controlinformation to be transmitted, among a plurality of pieces of controlinformation required for MU-MIMO transmission, common controlinformation commonly required to all STAs and dedicated controlinformation individually required to the STAs may be transmitted bydistinguishing the two types of information.

A PPDU format used in the WLAN system supporting MU-MIMO will bedescribed with reference to FIG. 5.

FIG. 5 shows an example of a PPDU format used in a next generation WLANsystem.

Referring to FIG. 5, a PPDU 500 includes an L-STF field 510, an L-LTFfield 520, an L-SIG field 530, a VHT-SIGA field 540, a VHT-STF field550, a VHT-LTF field 560, a VHT-SIGB field 570, and a data field 580.

A PLCP sub-layer converts a PSDU delivered from a MAC layer into thedata field 580 by appending necessary information to the PSDU, generatesthe PPDU 500 by appending several fields such as the L-STF field 510,the L-LTF field 520, the L-SIG field 530, the VHT-SIGA field 540, theVHT-STF field 550, the VHT-LTF field 560, the VHT-SIGB field 570, or thelike, and delivers the PPDU 500 to one or more STAs through a PMD layer.

The L-STF field 510 is used for frame timing acquisition, automatic gaincontrol (AGC) convergence, coarse frequency acquisition, etc.

The L-LTF field 520 is used for channel estimation for demodulation ofthe L-SIG field 530 and the VHT-SIGA field 540.

The L-SIG field 530 is used when an L-STA receives the PPDU to acquiredata.

The VHT-SIGA field 540 includes control information for interpreting thereceived PPDU 500 as common control information required for VHT-STAswhich are MIMO-paired with an AP. The VHT-SIGA field 540 includesinformation on a spatial stream for each of the plurality of MIMO-pairedSTAs, bandwidth information, identification information related towhether space time block coding (STBC) is used, a group identifier foran STA group, information on an STA to which each group identifier isallocated, and information related to a short guard interval (GI).Herein, the group identifier for the STA group may include whether acurrently used MIMO transmission method is MU-MIMO or SU-MIMO.

The VHT-STF field 550 is used to improve performance of AGC estimationin MIMO transmission.

The VHT-LTF field 560 is used when the STA estimates a MIMO channel.Since the VHT WLAN system supports MU-MIMO, the VHT-LTF field 560 can beconfigured by the number of spatial streams in which the PPDU 500 istransmitted. In addition, when full channel sounding is supported and isperformed, the number of VHT-LTFs may increase.

The VHT-SIGB field 570 includes dedicated control information requiredwhen the plurality of MIMO-paired STAs receive the PPDU 500 to acquiredata. Therefore, the STA may be designed such that the VHT-SIGB field570 is decoded only when the common control information included in theVHT-SIGB field 570 indicates that the currently received PPDU 500 istransmitted using MU-MIMO transmission. On the contrary, the STA may bedesigned such that the VHT-SIGB field 570 is not decoded when the commoncontrol information indicates that the currently received PPDU 500 isfor a single STA (including SU-MIMO).

The VHT-SIGB field 570 includes information on each STA's modulation,encoding, and rate-matching. A size of the VHT-SIGB field 570 may differaccording to the MIMO transmission method (MU-MIMO or SU-MIMO) and achannel bandwidth used for PPDU transmission.

A term ‘data block’ can be used as a general expression for the depictedPPDU. In addition, in a data field included in the data block, a parttransmitted to each of the MIMO-paired STAs can be expressed as a dataunit.

In order for a next generation WLAN system supporting MU-MIMO totransmit the same-sized PPDU to the STAs paired with the AP, informationindicating a bit size of the data field constituting the PPDU and/orinformation indicating a size of a bit-stream constituting a specificfield in the specific field may be included in the VHT-SIGA field.However, a field required in the L-SIG field may be used for aneffective PPDU format. That is, a length field and a rate field whichare transmitted by being included in the L-SIG field may be used toprovide necessary information so that the same-sized PPDU is transmittedto all STAs. In this case, additional padding may be required in a PHYlayer since an MPDU and/or an A-MPDU are configured on a byte (or octet)basis in a MAC layer. Hereinafter, a padding method will be described ingreater detail with reference to the accompanying drawings when the PPDUis generated in the PHY layer.

FIG. 6 shows a PPDU format according to an embodiment of the presentinvention. Although it is shown in FIG. 6 that the number of STAsreceiving a corresponding PPDU is limited to 3 and the number of spatialstreams allocated to each STA is 1, there is no particular limitation onthe number of STAs paired with an AP and the number of spatial streamsallocated to each STA.

Referring to FIG. 6, a PPDU 600 includes an L-STF field 605, an L-LTFfield 610, an L-SIG field 620, a VHT-SIGA field 630, a VHT-LTF field640, a VHT-SIGB field 650, a service field 660, a PSDU 670, a paddingfield 680, and a tail field 690. The L-STF field 605, the L-LTF field610, the L-SIG field 620, the VHT-SIGA field 630, the VHT-LTF field 640,and the VHT-SIGB field 650 are the same as those described above withreference to FIG. 5, and thus detailed descriptions thereof will beomitted. In addition, the PSDU 670 may be an MPDU and/or an A-MPDU whichare delivered from a MAC layer. A PSDU1 671 intended to be transmittedto an STA1, a PSDU2 intended to be transmitted to an STA2, and a PSDU3673 intended to be transmitted to an STA3 may have different bit sizesas illustrated.

In FIG. 6, the number of OFDM symbols required for transmission of datato each of a plurality of target STAs on the basis of MU-MIMOtransmission is indicated by a PPDU duration N_(SYM). Referring thefigure, all PPDU durations corresponding to each of the plurality oftarget STAs are same. The PPDU duration can be optionally re-defined asthe number of OFDM symbols required to transmit a data field. In thatcase, all the number of OFDM symbols for the data fields intended to betransmitted to each of the plurality of target STAs are same. However,the PPDU duration used in FIG. 6 will be used in the present invention.

Information indicating the PPDU duration may be included in the L-SIGfield 620. In the PPDU, the PPDU duration indicated by the L-SIG fieldincludes a symbol to which the VHT-SIGA field is allocated, a symbol towhich the VHT-LTF field is allocated, a symbol to which the VHT-SIGBfield is allocated, a bit (or bits) constituting the service field, bitsconstituting a PSDU, bits constituting the tail field, and bitsconstituting the padding field. A STA that receives a PPDU can acquireinformation about PPDU duration of the PPDU through the informationindicating the PPDU duration included in the L-SIG field.

In this case, a transmitting-end AP and/or STA for generating a PPDU andtransmitting the PPDU to a receiving-end STA may know information on thenumber N_(SYM) of symbols corresponding the PPDU duration, the numberN_(VHT-SIGA) of symbols to which the VHT-SIGA field is allocated, thenumber N_(VHT-LTFs) of symbols to which the VHT-LTF field is allocated,the number N_(VHT-SIGB) of symbols to which the VHT-SIGB field isallocated, 16 bits constituting the service field, and the number ofbits constituting the tail field. On the basis of this information, aPSDU to be allocated to an STAi and the number N_(VHT-PSDU+Pad,i) ofbits to be allocated to the padding field to be appended the PSDU can beknown. This can be expressed by Equation 6 below.

$\begin{matrix}\begin{matrix}{N_{{{VHT} - {PSDU} + {Pad}},i} = {\underset{\underset{{term} - 1}{}}{\begin{pmatrix}{N_{{VHT} - {SYM}} - N_{{VHT} - {SIGA}} -} \\{N_{{VHT} - {LTF}_{i}} - N_{{VHT} - {SIGB}}}\end{pmatrix} \times N_{{DBPS},i}} -}} \\{\underset{\underset{{term} - 2}{}}{( {16 + {6 \times N_{{BS},i}}} )}} \\{= {\underset{\underset{{term} - 1}{}}{N_{DATAfield} \times N_{{DBPS},i}} - \underset{\underset{{term}\; 2}{}}{( {16 + {6 \times N_{{BS},i}}} )}}}\end{matrix} & \lbrack {{Equation}\mspace{14mu} 6} \rbrack\end{matrix}$

N_(VHT-PSDU+Pad,i) denotes the total number of bits obtained byappending the number of bits allocated to a PSDU to be transmitted tothe STAi and the number of bits of the padding field. N_(DATAfield)denotes the number of bits of the data field. Since the MPDU and/or theA-MPDU are padded on an octet (or byte) basis, the number of bits of thePSDU (i.e., PSDU1, PSDU2, and PSUD3) may be a multiple of 8. Therefore,if a PSDU length (on an octet basis) can be known, the number of bitsallocated to the PSDU can be known. The number N_(Pad) of padding bitsallocated to the padding field can be expressed by Equation 7 below.

$\begin{matrix}\begin{matrix}{N_{{Pad},i} = {{N_{DATAfield} \times N_{{DBPS},i}} -}} \\{{N_{{{VHT} - {PSDU}},i} - ( {16 + {6 \times N_{{BS},i}}} )}} \\{= {{N_{DATAfield} \times N_{{DBPS},i}} - {8 \times {{PSDU\_}{length}}_{,i}}}} \\{{- ( {16 + {6 \times N_{{BS},i}}} )}}\end{matrix} & \lbrack {{Equation}\mspace{14mu} 7} \rbrack\end{matrix}$

The equation above shows that a bit size of the padding filed to beapplied may vary depending on a length of the PSDU to be transmitted toeach of the MU-MIMO paired STAs.

Referring to Equation 7, an AP and/or an STA which intend to generate aPPDU and transmit the PPDU to the plurality of MIMO-paired STAs candetermine the bit size of the padding field if a value of N_(DATAfield)can be known. In practice, the number of OFDM symbols required totransmit the PSDU to each of the MIMO-paired STAs may differ. This isbecause the value of N_(DATAfield) can be determined based on the lengthof the PSDU intended to be transmitted to the STA. On the other hand,the number N_(DATAfield) of OFDM symbols for the data field constitutingthe PPDU may be a specific number. Therefore, a transmitting-end ATand/or STA may determine the greatest value of the numberN_(DATAfield,i) of OFDM symbols required to transmit the PSDU to each ofthe MIMO-paired STAs as the number N_(DATAfield) of OFDM symbols for thedata field constituting the PPDU This can be expressed by Equation 8below.

N _(DATAfield)=max{N _(DATAfield,i)}_(i=1) ^(k)   [Equation 8]

Herein, N_(DATAfield,i) denotes the number of OFDM symbols required totransmit the PSDU to the STAi, and may be the number of OFDM symbols ofthe data field configured when generating a PPDU to be transmitted tothe STAi by using SU-MIMO or single-antenna transmission. NDATAfield,imay be determined based on information about the length of PSDU includedin TXVECTOR primitive(s). k denotes the total number of STAs which areMIMO-paired with the AP and/or the STA. Therefore, k may have an integervalue.

In another method, N_(VHT-PSDU+Pad) must be provided on a byte basis inorder to support the PPDU format of FIG. 6. That is, a padding procedureperformed in a PHY layer has to be performed on a last transmitted OFDMsymbol. This may occur when the number of bits in the term-1 of Equation6 above is not a multiple of 8. For example, assume that N_(DBPS) can beused in an extended manner in the HT WLAN system. In this case, apossible N_(DBPS) value is an even number, but a value which is not amultiple of 8 exists such as 26, 52, 78, 156, etc.(N_(VHT-SYM)−N_(VHT-SIGA)−N_(VHT-LTFs)−N_(VHT-SIGB)) must be an evennumber greater than or equal to 4. That is, if(N_(VHT-SYM)−N_(VHT-SIGA)−N_(VHT-LTFs)−N_(VHT-SIGB)) is an odd/primarynumber, the term-1 is not provided on a byte basis. In addition, in theterm-2, there may also a case where the tail field in which the numberof allocated bits is based on a multiple of 6 is not a multiple of 8. Inorder to set the N_(VHT-PSDU+Pad) to the multiple of 8 by consideringthe term-1 and the term-2, zero padding may be supported on a bit basisin the PHY layer. A detailed method of calculating the valueN_(VHT-PSDU+Pad) can be performed as shown in Equation 9 below.

N _(VHT-Pad)=(N _(VHT-PSDU+Pad))mod 8   [Equation 9]

According to Equation 9 above, the value N_(VHT-PSDU+Pad) may be one of0, 2, 4, and 6. This is because the value N_(SD) has an even number ingeneral and thus the value N_(DBPS) is also an even number.

A method of determining a bit size of a padding bit to be appended tothe aforementioned PPDU may be independently applied to each STA forreceiving the PPDU to be subjected to beamforming and transmission. Thisis because a size of the tail field may differ from one STA to anothersince there is a difference in the number of spatial streams allocatedto each of the MIMO-paired STAs, a bit size of data to be transmitted(i.e., a bit size of the PSDU), and the number of BCC encoders used forgeneration of the PPDU.

FIG. 7 shows an example of a transmission (Tx) procedure based on a PPDUgeneration method according to an embodiment of the present invention.

Referring to FIG. 7, a MAC layer delivers a generated MPDU or A-MPDU toa PLCP sub-layer. In the PLCP sub-layer, the MPDU or A-MPDU is called aPSDU. The PLCP sub-layer transmits the PSDU to a different STA through aPHY layer, and appends necessary control information so that thedifferent STA receives the PPDU and acquires data by performingdemodulation and decoding. The control information may be included in anL-SIG field, a VHT-SIGA field, and a VHT-SIGB field, and a tail fieldfor indicating an encoder type (in case of a BCC encoder) may beadditionally appended. The control information included in the VHT-SIGAfield and VHT-SIGB field may be the control information mentioned abovewith reference to FIG. 5. The PLCP sub-layer may further append atraining symbol for radio resource synchronization, timing acquisition,antenna diversity acquisition, or the like between a transmitting-end APand/or STA and a receiving-end STA. This can be implemented by appendinga legacy preamble including an L-STF and L-LTF for an L-STA, and aVHT-STF and VHT-LTF for a VHT-STA. A PPDU transmitted through a radioresource is transmitted by being mapped to an OFDM symbol. Herein, thePPDU mapped to the OFDM symbol and/or a data field included in the PPDUmay be implemented to have a specific size, and can be implemented witha multiple of an octet number as described above. Therefore, in order tomatch to a bit size of the PPDU and/or data field, if the bit size isnot sufficient, a padding bit may be appended to match to a total bitsize of the PPDU and/or the data field. In this case, a detailed methodof appending the padding bit can be implemented based on the embodimentsof FIG. 5 to FIG. 7 and Equation 6 to equation 8. When the bit size ofthe PPDU and/or the data field is provided on an octet basis, a lengthof the padding field appended thereto may be 0 to 7 bits. The PPDUgenerated in the Tx procedure includes a preamble for the L-STA andVHT-STA, an L-SIG field including control information required for theL-STA, a VHT-SIG (i.e., VHT-SIGA and VHT-SIGB) field including controlinformation required for the VHT-STA, and a data field including aservice field, a PSDU field, and a tail field. The generated PPDU may bemapped to OFDM symbols, and then may be transmitted to at least one ormore target STAs which are MIMO-paired.

FIG. 8 shows an example of a reception (Rx) procedure for a generatedPPDU according to an embodiment of the present invention.

Referring to FIG. 8, upon receiving the PPDU, a receiving-end STA canevaluate a size of an appended padding bit according to theaforementioned equation. Therefore, a PHY layer of the receiving-end STAcan determine whether there is a bit constituting a padding fieldincluded in a data field of the received PPDU, and if it exists, candelete the padding bit. Thereafter, the receiving-end STA can acquirethe transmitted data by transmitting a PSDU to a MAC layer.

FIG. 9 is a block diagram showing a wireless apparatus for implementingan embodiment of the present invention.

Referring to FIG. 9, a wireless apparatus 900 includes a processor 910,a memory 920, and a transceiver 930. The transceiver 930 transmitsand/or receives a radio signal, and implements an IEEE 802.11 physicallayer. The processor 910 is operationally coupled to the transceiver 930and is configured to implement a MAC layer and/or PHY layer forimplementing the embodiment of the present invention shown in FIG. 5 toFIG. 8 related to a PPDU generation method.

The processor 910 and/or the transceiver 930 may include anapplication-specific integrated circuit (ASIC), a separate chipset, alogic circuit, and/or a data processing unit. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory 920 and may be performed by the processor 910. The memory 920 maybe located inside or outside the processor 910, and may be coupled tothe processor 910 by using various well-known means.

A specific padding bit is appended to data intended to be transmitted tostations (STAs) paired using multi user-multiple input multiple output(MU-MIMO) so as to generate PPDUs having the same bit size. Accordingly,a plurality of pieces of different-sized data can be transmitted throughthe same number of orthogonal frequency division multiplexing (OFDM)symbols to each STA by using MU-MIMO transmission.

What is claimed is:
 1. A method for receiving a data block in a wirelesscommunication system, the method performed by a receiver and comprising:receiving a physical layer protocol data unit (PPDU) from a transmitterover an operating channel, the PPDU including a signal field, a VeryHigh Throughput-Signal-A (VHT-SIG-A) field, a Very HighThroughput-Signal-B (VHT-SIG-B) field and a padded data block,generating a first data block by removing zero or more physical paddingbits from the padded data block in a physical layer; and generating asecond data block by removing zero or more Medium Access Control (MAC)padding bits from the first data block in a MAC layer, wherein a numberof bits for the zero or more physical padding bits is smaller than oneoctet, wherein the signal field includes information used forcalculating a number of bits for the zero or more MAC padding bits,wherein the VHT-SIG-A field includes group information indicating thereceiver to receive the PPDU, wherein the VHT-SIG-B field includesinformation indicating the length of the second data block, and whereina number of bits for the VHT-SIG-B field is determined based on abandwidth of the operating channel.
 2. The method of claim 1, whereinthe VHT-SIG-A field includes a bandwidth field indicating the bandwidthof the operating channel.
 3. The method of claim 1, wherein the PPDUfurther includes a Very High Throughput-Short Training Field (VHT-STF)and a Very High Throughput-Long Training Field (VHT-LTF).
 4. The methodof claim 1, wherein the number of bits for the zero or more physicalpadding bits is determined by:N _(PAD) ={Nsym N _(DBPS)−8 PSDU_(length)−(16+6N _(ES))}mod 8 wherein:N_(PAD) indicates the number of bits for the zero or more physicalpadding bits and ranges from 0 to 7; Nsym indicates a number oforthogonal frequency division multiplex (OFDM) symbols corresponding tothe length of the first data block; N_(DBPS) indicates a number of databits per symbol; PSDU_(length) indicates the length of the first datablock; N_(ES) indicates a number of binary convolution coding (BCC)encoders, and ‘mod’ denotes modulo operation.
 5. A device for wirelesscommunication system comprising: a processor; and a transceiveroperatively coupled to the processor and configured to transmit andreceive a data block, wherein the processor having a Medium AccessControl (MAC) layer and a physical (PHY) layer is configured to:instruct the transceiver to receive a physical layer protocol data unit(PPDU) from a transmitter over an operating channel, the PPDU includinga signal field, a Very High Throughput-Signal-A (VHT-SIG-A) field, aVery High Throughput-Signal-B (VHT-SIG-B) field and a padded data block,generate a first data block by removing zero or more physical paddingbits from the padded data block in the PHY layer; and generate a seconddata block by removing zero or more MAC padding bits from the first datablock in the MAC layer, wherein a number of bits for the zero or morephysical padding bits is smaller than one octet, wherein the signalfield includes information used for calculating a number of bits for thezero or more MAC padding bits, wherein the VHT-SIG-A field includesgroup information indicating the device to receive the PPDU, wherein theVHT-SIG-B field includes information indicating the length of the seconddata block, and wherein a number of bits for the VHT-SIG-B field isdetermined based on a bandwidth of the operating channel.
 6. The deviceof claim 5, wherein the VHT-SIG-A field includes a bandwidth fieldindicating the bandwidth of the operating channel.
 7. The device ofclaim 5, wherein the PPDU further includes a Very High Throughput-ShortTraining Field (VHT-STF) and a Very High Throughput-Long Training Field(VHT-LTF).
 8. The device of claim 5, wherein the number of bits for thezero or more physical padding bits is determined by:N _(PAD) ={Nsym N _(DBPS)−8PSDU_(length)−(16+6N _(ES))}mod 8 wherein:N_(PAD) indicates the number of bits for the zero or more physicalpadding bits and ranges from 0 to 7; Nsym indicates a number oforthogonal frequency division multiplex (OFDM) symbols corresponding tothe length of the first data block; N_(DBPS) indicates a number of databits per symbol; PSDU_(length) indicates the length of the first datablock; N_(ES) indicates a number of binary convolution coding (BCC)encoders, and ‘mod’ denotes modulo operation.